This disclosure relates generally to the field of neutron activation spectroscopy of subsurface formations. More specifically, the disclosure relates to methods for determining elemental weight fractions of subsurface formations using both capture gamma rays and inelastic gamma rays resulting from neutron activation of the subsurface formations.
Nuclear spectroscopy performed within wellbores drilled through subsurface formations may provide estimates of the chemical composition of one or more of the formations. For chemical composition analysis, nuclear spectroscopy is generally divided into two classes nuclear spectroscopy of inelastic neutron collision measurements, and nuclear spectroscopy of thermal neutron capture measurements. In particular, when a formation is bombarded with high energy neutrons (e.g., 14 MeV), from a neutron source deployed in the wellbore, some of the neutrons inelastically scatter upon collision with the nuclei of certain atoms in the formations and as a result generate gamma rays having characteristic energy spectra related to the particular atoms with which the neutrons collide.
A wellbore spectroscopy tool may include a so called pulsed neutron generator (PNG) as a source. A PNG emits controlled duration “bursts” of high energy neutrons. Gamma rays may be detected in selected time intervals (“windows”) referenced to the time during which the neutrons are being generated. Detection while the neutrons are being generated may be used to measure the spectrum of gamma ray energies, particularly inelastic in such case. The gamma ray energy spectrum can then be analyzed using a set of pre-defined elemental standard spectra to determine the relative contribution of each element to the measured spectrum. Elements typically included in an inelastic spectrum include carbon (C), oxygen (O), silicon (Si), calcium (Ca), iron (Fe) and sulfur (S) among others. The most common application for inelastic spectroscopy data is to use a carbon to oxygen ratio to estimate formation water saturation (fractional volume of formation pore space that is water filled), although the results of inelastic gamma ray measurements have also been used in determining formation mineral composition (lithology). See, e.g., U.S. Pat. No. 5,440,118 to Roscoe which is hereby incorporated by reference herein in its entirety.
Similarly, when neutrons from any source, such as a PNG, a radioisotope source or other source, bombard a formation, the neutrons eventually lose energy until they reach thermal level (i.e., where their motion is substantially related to ambient temperature). At thermal energies neutrons may be captured by the nuclei of certain formation elements, upon which the capturing nuclei emit gamma rays having energies that are characteristic of the specific element. Again, a wellbore spectroscopy tool may be used to detect the capture gamma rays. Such detection ordinarily takes place in a later time window when a PNG is used, and the gamma ray spectrum may be analyzed to determine the relative contributions of each of the contributing elements to the measured gamma ray spectrum. Elements in a capture gamma ray spectrum may include, for example and without limitation, silicon (Si), calcium (Ca), iron (Fe), sulfur (S), titanium (Ti), gadonlinium (Gd), hydrogen (H), chlorine (Cl), aluminum (Al), sodium (Na), magnesium (Mg), manganese (Mn), nickel (Ni) and phosphorus (P) among others. The contributions of the various elements to the gamma ray spectrum may then be used to estimate elemental concentrations through a geological model, sometimes referred to as “oxides closure”. See. Gum et al., 1989, A Geological Model for Gamma-ray Spectroscopy Logging Measurements, Nucl. Geophysics, Vol. 3, No. 4, pp. 351-359 and U.S. Pat. No. 4,810,876 issued to Wraight et al, which is hereby incorporated by reference herein in its entirety.
U.S. Pat. No. 7,366,615 issued to Herron et al. describes a method for calibrating the elemental spectral yields from inelastic reactions using a single element common to both capture and inelastic reactions. The method disclosed in Herron et al. '615 works best where sufficient silicon is present. Also, the method disclosed in Herron et al. '615 does not include combining the concentration estimates to produce enhanced concentration estimates for all of the elements measured using both capture and inelastic gamma ray spectroscopy.
There exists a need for improved techniques for determining elemental concentrations from neutron activation measurements.